Sea of Worlds: The Anunnaki

Roy Masterson is a wounded American veteran that fought in Afghanistan. He is conscripted into a time war by the ubiquitous Genitor. His consciousness is placed in the body of an alien humanoid known as a Dionan, then forced into military service and trained on the planet Cassida. Masterson is deployed to fight on a planet called Mundas by the Genitor and their Cassidan allies, but he comes to realize it's his own planet Earth in the distant past.